1. Field of the Invention
The present invention relates to a color image processing apparatus and a method for processing a color image for correcting colors of an image output from a printer.
2. Description of the Related Art
In these years, as the performance of electrophotographic apparatuses improves, machines that realize the same image quality as printers are being developed. However, there is still a problem in that the amount of variation in color is larger than that of printers due to instability unique to electronic photographs. Therefore, various calibration techniques are incorporated into existing electrophotographic apparatuses.
Some of the existing electrophotographic apparatuses are provided with a calibration technique in which one-dimensional tone correction lookup table (LUT) corresponding to cyan, magenta, yellow, and black (C, M, Y, and K) toner is created in order to correct primary colors. A LUT is a table indicating output data corresponding to input data divided at certain intervals, and is capable of expressing nonlinear characteristics that are hard to express with an arithmetic expressions. When calibration for these “single colors” (hereinafter referred to as “single-color calibration”), each of which indicates a color expressed by the C, M, Y, or K toner, have been executed, the reproduction characteristics of the single colors such as maximum densities and tone are corrected.
In addition, recently, a technique for performing calibration for “multi-colors” using a four-dimensional LUT has been proposed in Japanese Patent Laid-Open No. 2011-254350. Here, the “multi-colors” refer to colors for each of which a plurality of colors of toner are used, such as red, green, and blue for which two colors among C, M, and Y are used and gray for which C, M, and Y are used. In particular, in the case of electronic photographs, even if the tonal characteristics of the single colors are corrected using a one-dimensional LUT, nonlinear differences are likely to be generated when the “multi-colors” are expressed using a plurality of colors of toner. Here, if calibration for the multi-colors is executed, the color reproduction characteristics of the multi-colors expressed as combinations (overlaps or the like) between a plurality of colors of toner are corrected.
A procedure of calibration including the “multi-colors” will be described. First, patch images are printed on a recording medium such as a sheet of paper using chart data including the single colors in order to perform calibration for the “single colors”. Each of the patch images is an image for measuring a color having a uniform density and a certain area. A pattern image is obtained by generating a plurality of patch images whose colors are different and printing the generated patch images on a recording medium. The recording medium such as a sheet of paper on which the pattern image is printed is read by a scanner or a sensor to read the patch images. Data obtained by reading the patch images is compared with predetermined target values, and a one-dimensional LUT for correcting differences from the target values is created. Next, patch images are printed on a recording medium using chart data including the multi-colors that reflect the one-dimensional LUT created before in order to perform the calibration for the “multi-colors”, and the patch images are read by the scanner or the sensor. Data obtained by reading the patch images are compared with predetermined target values, and a four-dimensional LUT for correcting differences from the target values is created.
In an image processing apparatus, the values of the multi-colors vary in a nonlinear manner due to various factors caused during a process for forming an image. For example, toner transferred to a transfer belt in a secondary transfer can come off before next toner is transferred. It is difficult to predict the amount of toner that comes off because the amount does not depend on the amount of toner transferred. Therefore, the values of the multi-colors vary in a nonlinear manner. In order to correct the nonlinear variation in the values of the multi-colors, multi-color calibration is executed. Correction values obtained by executing the multi-color calibration are created in order to minimize color differences between colorimetric values and target values in a Lab space. Therefore, when a difference between a colorimetric value and a target value for a certain color is large in the Lab space, the mixing ratio of C, M, Y, and K that form the color is changed and the value in the Lab space is smoothed in order to minimize the difference. As a result, the colorimetric value for the certain color becomes close to the target value, and the correction seems to have been appropriately performed. When the amount of change in the ratio is large, however, the balance between the mixing ratios of C, M, Y, and K of a grid point indicating a certain color in the Lab space and a grid point located close to the aforementioned grid point is lost. In addition, since smoothing in CMYK values is not performed, continuity is not secured. That is, whereas the color value of each color becomes close to a target value in the Lab space, the continuity of the mixing ratios of C, M, Y, and K between a color to be paid attention to and surrounding colors can be lost. Thus, when the amount of correction performed using these correction values becomes large, it is likely that the continuity of tone is lost in a smooth image printed using results of the correction, such as a gradation image.
In addition, one of the reasons why the amount of correction obtained by executing the multi-color calibration becomes large is that the reproduction characteristics of the multi-colors have been significantly deviated from reference characteristics due to deterioration over time.
As described above, when a process for correcting the reproduction characteristics is executed when a color image is formed, the continuity of tone in the image can be lost if correction values obtained by executing the multi-color calibration are used, and accordingly an image desired by a user might not be reproduced.